1. Field of the Invention
The present invention relates generally to a technique for operation of a nickel-hydrogen battery, and more particularly, to recharging a nickel-hydrogen battery at a temperature which is substantially lower than the temperature at which discharge is performed, automatically correcting errors in the total charge returned to the battery during recharge to thereby establish the ideal battery charge and temperature leading into the next discharge cycle.
2. Description of the Prior Art
Commonly assigned U.S. Pat. No. 5,395,706 entitled xe2x80x9cSatellite Battery Thermal/Capacity Designxe2x80x9d and U.S. Pat. No. 5,429,888, entitled xe2x80x9cBattery Recharging Techniquexe2x80x9d both relate to recharging a nickel-hydrogen battery at a temperature which is substantially lower than the temperature at which discharge is performed. The disclosures of these patents are incorporated into the instant disclosure in their entirety by reference. According to these patents, the charging operation uses preset high, medium, taper, and pulse charge rates. The battery""s temperature is made to follow a prescribed temperature profile throughout. Beginning at the transition from high to medium charging, heater power is required to keep the batteries at their cold charge temperature setpoint, which typically is xe2x88x9220xc2x0 C. In charging the battery, about half way through taper charge, the heater power falls to zero. This marks a transition in the battery charging thermodynamics and roughly corresponds to the point at which the batteries reach a substantial fraction of their total charge capacity. Note that throughout the heater power transition, the battery""s temperature remains at the setpoint of 20xc2x0 C. From the heater power transition onward, much of the charge power is dissipated as heat, rather than absorbed as useful energy capacity. There is, therefore, a slight rise in the battery temperature during the last portion of taper charging. At the end of taper charging, the temperature setpoint is changed to xe2x88x9215xc2x0 C. (typically) in preparation for battery discharge. There is an associated rise in heater power after the end of taper charge.
The prior art in the form of the U.S. Pat. Nos. 5,395,706 and 5,429,888 just described is based on a feed-forward controller which does not use information about the battery""s temperature, voltage, or pressure during recharge. Instead, it simply applies charge energy to the battery in proportion to the energy lost by the battery during the most recent discharge.
The primary advantage of the present invention over the prior art is that it eliminates the labor Intensive and risky manual tuning process required to achieve optimal battery charge and thermal behavior. The prior art cited above has no ability and makes no attempt to automatically correct errors in the total charge returned to a battery during recharge. Such errors in the total charge arise inevitably from the power control electronics surrounding the battery, including battery charge and discharge current monitor calibration and measurement errors, and charge controller errors, Errors also arise from uncertainties in the theory of thermal dissipation and charge acceptance efficiency that leads to the prescribed charge currents. For digital systems, errors also arise from discretization of the total charge to be returned. Furthermore, the battery temperature near the end of recharge is very sensitive to these errors. As a result of this sensitivity, with the prior art the particular charge control parameters must be manually tuned in order to achieve adequate performance for each spacecraft. This tuning process is labor intensive and, for satellite applications, risky since the tuning process can only be done once the satellite is experiencing actual eclipses on orbit. Furthermore, in the prior art one manual tuning procedure may not be adequate over the spacecraft""s entire life due to drifting electronic component performance.
It was with knowledge of the foregoing state of the technology that the present invention has been conceived and is now reduced to practice.
The present invention relates to a method of operating a rechargeable battery having a nickel hydroxide positive electrode and an electrolyte. This method comprises the steps of charging the battery, after completion of a discharge cycle, at a temperature T1 between approximately xe2x88x9210xc2x0 C. and xe2x88x9230xc2x0 C. which is lower than a temperature T2, in the range of approximately xe2x88x9210xc2x0 C. to +5xc2x0 C., at which discharge begins, automatically determining the total charge to be returned to the battery for establishing the ideal charge and temperature for the battery leading into the next discharge cycle, and applying charge energy to the battery having the magnitude as automatically determined. After completion of the discharge cycle, the battery is cooled to the temperature in the T1 range, then heated to stabilize the temperature to that in the T1 range. Then the battery is charged according to a nominal profile of charge power as a function of time, the accumulated charge imparted to the battery sensed as cooling proceeds, the nominal power profile adjusted according to the accumulated charge obtained, and the remainder of the nominal charge profile adjusted accordingly. More specifically, the battery is heated according to a nominal profile of heater power as a function of time which includes an upward transition portion for initializing the heating operation, a plateau portion for maintaining the heating operation, and a downward transition portion for terminating the heating operation, and the actual downward transition in heater power obtained is sensed, and the nominal power profile is adjusted according to the time of the actual downward transition sensed. This results in decreasing the total charge energy returned to the battery in the event the downward transition operation sensed actually occurs prior to the nominal transition time and results in increasing the total charge returned to the battery in the event the downward transition operation sensed actually occurs after the nominal transition time operation.
A primary feature, then, of the present invention is the provision of an improved technique for operation of a nickel-hydrogen battery.
Another feature of the present invention is the provision of such a technique for recharging a nickel-hydrogen battery at a temperature which is substantially lower than the temperature at which discharge is performed and automatically correcting errors in the total charge returned to the battery during recharge to thereby establish the ideal battery charge and temperature leading into the next discharge cycle.
A further feature of the present invention is the provision of such a technique which incorporates use of temperature controller information in a nickel hydrogen battery charge controller.
Still another feature of the present invention is the provision of such a technique which automatically tunes the total charge returned to the battery thereby correcting all of the error sources to which the battery is subjected, thereby establishing the ideal battery charge and temperature leading into the next discharge cycle.
Yet another feature of the present invention is the provision of such a technique which renders the known labor intensive and risky manual tuning process largely obsolete.
Yet another feature of the present invention is the provision of such a technique which eliminates the possibility of overcharging, and hence, overheating, nickel-hydrogen batteries during recharge after discharge and which, in turn, may lengthen the battery""s operational lifetime.
Still another feature of the present invention is the provision of such a technique which, for satellite applications, serves to improve the satellite""s overall lifetime and reliability.
Still a further feature of the present invention is the provision of such a technique which eliminates the possibility of undercharging nickel-hydrogen batteries during recharge after discharge which, in turn, protects against (1) inadequate battery energy capacity in the next discharge period and (2) catastrophic battery failure due to cell fusion and, for satellite applications, protects against forced payload turnoffs, or load shedding, and prevents service interruptions.
Yet a further feature of the present invention is the provision of such a technique which greatly reduces the labor required to tune charge control parameters, using the same parameter set for all batteries of a given capacity and thermal environment, for satellite applications, there being one parameter set for all spacecraft in a given class and there being no need to manually tune each spacecraft""s battery software parameters after launch, a risky and costly procedure, and there being no need to retune the software parameters as the spacecraft ages.
Still another feature of the present invention is the provision of such a technique which uses a temporal setpoint for a temperature controller output transition and subsequent temporal shift of charge currents to optimally adjust total charge returned to the battery and the battery""s temperature before the next discharge period.
Other and further features, advantages, and benefits of the invention will become apparent in the following description taken in conjunction with the following drawings. It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory but are not to be restrictive of the invention. The accompanying drawings which are incorporated in and constitute a part of this invention, illustrate one of the embodiments of the invention, and together with the description, serve to explain the principles of the invention in general terms. Like numerals refer to like parts throughout the disclosure.